Cellular wireless communications have been long established in the field. So-called macro-cells are defined by the effective range of operation of base stations deployed in an area of coverage. By spacing base stations apart, a pattern of macro-cells can be established.
To meet increases in traffic demand, it is known to split existing macro-cells by provision of further base stations, leading to smaller macro-cells. Additionally, a single macro-cell may in fact be generated by angularly spaced antennas, each covering a portion of the coverage region. In existing implementations, three antennas may be spaced 120° apart. These three antennas may be thought of as defining separate sub-regions of a cell.
If a new cellular technology were to be deployed from scratch, backwards compatibility with existing technologies and installations would not be required. In such a case, macro-cells might be deployed at a finer spacing than in existing deployments, thereby leading to smaller macro-cells.
These homogeneous arrangements all involve the provision of a plurality of macro-cells, all of substantially the same communications technology. Other communications technologies might be overlaid on the macro-cell deployment. For instance, an indoor environment might present particular technical challenge to a macro-cell deployment, in that EMC shielding effects and other interferences might preclude effective macro-cell coverage. Thus, a smaller scale indoor solution, overlaid on the macro-cell structure, may provide enhanced coverage.
Heterogeneous arrangements, for instance consisting of a macro-cell network, and smaller cells defined perhaps by lower power base stations, and perhaps of a different communications technology, can now be encountered.
While the smaller cells so provided can deliver improved spatial diversity, thereby enhancing system capacity, certain technical challenges can arise in certain circumstances. A notable issue is that of inter-cell interference, particularly where a user station (User Equipment, UE) is positioned at or near a boundary between two cells. In such a situation, the UE, operating in a particular cell, may experience interference from communications within another, adjoining cell. While this problem is known and well understood, its prevalence will be increased in the context of cells of reduced size, with correspondingly increased boundary situations.
One well known approach to mitigating inter-cell interference is the re-use of frequencies. One possible technique comprises the imposition of a rule that two adjacent cells use mutually orthogonal frequencies. This approach can, in general, completely eliminate inter-cell interference, but at the cost of lower spectrum efficiency.
Instead of using different frequencies per cell, fractional frequency reuse (FFR) involves dividing user terminals into two groups, central cell user terminals and cell edge user terminals. In FFR, frequency reuse is only employed for cell edge user terminals, as these are the only user terminals which risk encountering inter-cell interference. Central cell user terminals in adjacent cells can use the same frequency. FFR can improve spectrum efficiency and mitigate inter-cell interference, but it still has certain drawbacks.
Firstly, the frequencies used for cell edge user terminals are pre-determined, and it is difficult therefore to adapt to external factors, such as environmental conditions, or a change in the number of UEs classed as cell-edge user terminals (bearing in mind that a UE has the potential for mobility). This is because the interfaces between the base stations normally need to be delay tolerant, and also the interfaces can only support very low transmission rates, which makes it impractical to make frequent changes to the predefined frequencies used for cell edge user terminals.
Secondly, two schedulers are required in order to schedule central cell user terminals and cell edge user terminals separately.